This invention relates to electrical light emitting elements, and more specifically to light emitting elements each of which is composed of a metal-insulator-semiconductor junction (hereinafter called "MIS junction") and has good stability in dynamic characteristics and excellent luminescence efficiency.
Electrical light emitting elements, especially, visible light emitting diodes (LED) have found wide spread commercial utility as functional display elements in various fields. Recently, researchers have also been started to apply them to printers and minifacsimiles.
These days LEDs which emit red light, yellow light, green light or the like employ, as their substrates, compound semiconductors of Group III-V elements of the periodic table such as Gap, GaAsP, GaAlAs. A dopant, for example, zinc, oxygen or nitrogen is suitably implanted in such substrates to form p-n junctions so as to make use of the light emission through recombination at the interfaces.
Such LEDs have already been commercially available, because of their high luminescence efficiencies of 0.2 to several percents.
However, in the case of LEDs capable of emitting blue light, where the compound semiconductors of Group II-VI are preferably employed, it is difficult to form p-n junctions as good as those of Group III-V since the semiconductors have wide band gaps.
Reflecting the above-mentioned difficulties, considerable research effort has been devoted to developing blue LEDs formed by MIS junctions, where a thin insulating layer is incorporated into metal and semiconductors such as zinc sulfide, zinc selenide or the like.
The minority carrier injections, in principle, are markedly improved by using MIS junctions.
However, the reliability of these light emitting elements composed of MIS structures is significantly poor when compared with the above-described red LEDs and the like.
One of the reasons for such lower reliability is ascribed to the extreme difficulty in preparing good insulating thin films on the compound semiconductors, in contrast with silicon semiconductor substrates, which have excellent insulating films of silicon oxide.
When fabricating a light emitting element based on an MIS junction, the following are essential requirements for the interposed insulating film.
First, the insulating film must have a thickness thin enough to provide the tunnel effect for electrons or holes and must also be homogeneous and uniform without structural defects, and the thickness must be controlled in the order of 100 .ANG. of thinner. As a second requirement, no trap levels capable of capturing electrons and/or holes can exist in the film, the interface between the insulating film and metal layer or the interface between the insulating film and semiconductor substrate. Further, the insulating film must be free from deterioration when it is exposed to heat or a voltage of desired level.
In order to satisfy such requirements and to fabricate new electronic devices, it is attempted to deposit Langmuir-Blodgett films (hereinafter called "LB films") onto various semiconductor substrates by the Langmuir-Blodgett method (hereinafter called "LB method") and then to make use of the thus-applied LB films as insulating films. Thin organic films prepared by the LB method have attracted much attention compared with other film deposition methods such as evaporation, plasma polymerization and electrochemical deposition, since they have excellent film qualities and fulfill the above-described requirements. On the practical side, the selection of film-forming molecules in the LB method is of vital importance.
There are two reports on light emitting elements each of which makes use of the LB film as the insulating films of an MIS junction. One of the two reports is found in "Thin Solid Film", 99, 283 (1984).
This report deals with an MIS light emitting element with an LB film of cadmium stearate, one of the fatty acids, formed on an n-GaP semiconductor substrate. Since fatty acids are poor in mechanical strength, heat resistance and durability to applied voltages, they are not satisfactory for insulating films. However, their formation into LB films is easy and the thicknesses of the resulting LB films can be controlled on the order of the lengths of their molecules (in the case of stearic acid, about 25 .ANG. per layer). The above-mentioned light emitting element makes use of an LB film of such a fatty acid, and its luminous efficiency increases as the film thickness becomes larger, reaching the maximum at a film thickness of 200 to 250 .ANG..
However, the above light emitting element is accompanied by drawbacks such as the fact that its light emitting characteristics are unstable and less durable, and are thus reduced even when operated for a short period of time.
The other report is found in "Electronics Letters" 20, (12), 489-491 (1984).
This report is directed to an MIS light emitting element with an LB film of a phthalocyanine derivative deposited on an n-GaP semiconductor substrate. The luminescence efficiency of this light emitting element reaches the maximum value of 8.6.times.10.sup.-3 % at the LB film thickness of 57 .ANG.. The efficiency of the element is comparable to those of the above-mentioned elements composed of p-n junctions.
Moreover, this maximum luminescence efficiency is as good as that of MIS GaP LED having inorganic SiO.sub.2 films of 40 .ANG. thick as an insulating film.
Phthalocyanines applicable to the formation of the LB film have good thermal stability among various organic materials. Although the thermal and mechanical properties, in comparison with light emitting elements fabricated by using stearic acid, were improved, the thus-obtained light emitting element employing phthalocyanines still showed some undesired deterioration in the device characteristics during continuous operations. Hence, the proposed light-emitting element still involves many problems which have to be solved for its practical application.
For these reasons, there is an outstanding demand for the development of an insulating film which has fewer trap levels and still better characteristics from the viewpoint of stability with the passage of time, etc.